Expedient Mechanosynthesis of N,N-Dialkyl Imidazoliums and Silver(I)-Carbene Complexes in a Ball-Mill

Article abstract of DOI:10.1002/chem.201503472

The absence of solvent, associated with intensive mechanical agitation, allowed the first mechanosynthesis of high-value silver(I)-carbene complexes and the corresponding N,N-dialkylimidazolium precursors. This procedure gave outstanding results in terms of yield and reaction time, when compared to solution-based conditions previously described in literature, and was generalized to unprecedented compounds. Silver(I)-carbene complexes could either be obtained from N,N-dialkylimidazolium salts or directly from imidazole and alkyl halides in a one-pot two-step procedure without isolating the imidazolium intermediate. Additionally, an efficient one-pot three-step sequence, including imidazole alkylation, silver metalation, and transmetalation is reported. You only hit twice! Solvent-free mechanochemical conditions enabled the rapid synthesis of high-value N,N-dialkyl-substituted NHC silver(I) complexes and the corresponding N,N-dialkylimidazolium precursors. Mechano-metalation and transmetalation are performed in a highly user- and eco-friendly approach. By using this strategy, up to three consecutive chemical steps, leading to organometallic complexes, could be performed in a ball-mill with high efficiency.

Full text of DOI:10.1002/chem.201503472

DOI: 10.1002/chem.201503472
Communication
&
Mechanochemistry
Expedient Mechanosynthesis of N,N-Dialkyl Imidazoliums and
Silver(I)–Carbene Complexes in a Ball-Mill
Audrey Beillard, Ethan Golliard, Valentin Gillet, Xavier Bantreil,* Thomas-Xavier MØtro,*
Jean Martinez, and FrØdØric Lamaty*[a]
Dedicated to the memory of Dr. Guy Lavigne
mental impact of such reactions is to use solvent-free mecha-
nosynthesis.[4] While the benefits of mechanochemistry have al-
Abstract: The absence of solvent, associated with inten-
ready been demonstrated for the generation of organometallic
complexes,[5] the mechanosynthesis of N,N-dialkylimidazolium
halides and their corresponding [Ag(NHC)X] complexes has
never been reported. As an extension of our studies on mecha-
nochemistry for organic synthesis[6] and generation of Pd nano-
particles,[7] we detail herein the advantages of using a mecha-
nochemical approach for the production of widely used N,N-di-
alkylimidazolium salts and valuable [Ag(NHC)X] complexes
(Scheme 1).
sive mechanical agitation, allowed the first mechanosyn-
thesis of high-value silver(I)–carbene complexes and the
corresponding N,N-dialkylimidazolium precursors. This
procedure gave outstanding results in terms of yield and
reaction time, when compared to solution-based condi-
tions previously described in literature, and was general-
ized to unprecedented compounds. Silver(I)–carbene com-
plexes could either be obtained from N,N-dialkylimidazoli-
um salts or directly from imidazole and alkyl halides in
a one-pot two-step procedure without isolating the imida-
zolium intermediate. Additionally, an efficient one-pot
three-step sequence, including imidazole alkylation, silver
metalation, and transmetalation is reported.
In addition to their remarkable anticancer and antimicrobial
properties,[1] silver(I)–carbene complexes ([Ag(NHC)X]) are rec-
ognized as a universal platform for the generation of organo-
metallic complexes.[2] This can be explained both by the great
diversity of N-heterocyclic carbenes (NHCs) and a wide range
of metals that could be used. Besides, [Ag(NHC)X] complexes
bearing N,N-dialkyl-substituted NHCs have proved to have in-
teresting biological activities[1a] and electroluminescent proper-
ties.[3] However, [Ag(NHC)X] complexes suffer from chemo- and
photosensitivity, and their production requires long reaction
times and/or excess reagents. In the literature, their syntheses
are mostly performed in CH2Cl2, a highly volatile organic sol-
vent suspected of causing cancer as indicated by the GHS clas-
sification. The corresponding imidazolium halide precursors
were mostly obtained through alkylation of imidazoles, with
extended reaction times at refluxing temperatures. A conven-
ient way to increase the efficiency and to reduce the environ-
Scheme 1. Strategy for the mechanosynthesis of [Ag(NHC)X].
Initially, we focused on the synthesis of N,N-dialkylimidazoli-
um salts, which were obtained by nucleophilic substitution of
alkyl halides with N-alkyl imidazoles. Unless noted otherwise,
all reactions were realized without solvent in a 10 mL stainless
steel reactor containing a 10 mm diameter stainless steel ball
that was placed under agitation at 25 Hz in a vibratory ball-mill
(vbm). The reaction mixtures were recovered and treated with
preferable solvents, such as EtOH, iPrOH, or EtOAc/water.[8]
Subsequent filtration allowed elimination of any metallic parti-
cles and isolation of pure products.[6f] For comparison, the best
results reported in the literature for experiments performed in
solution have been included (Table 1).
The reaction of benzylbromide with N-methyl- and N-isopro-
pyl-imidazole yielded imidazoliums 1a·HBr and 1b·HBr, respec-
tively, in more than 98% within only 1 h (Table 1, entries 1 and
2). Interestingly, the synthesis of 1a·HBr was performed with
the same efficiency on a 4 g scale, using a 250 mL reactor agi-
tated in a planetary ball-mill (pbm). Bidentate ligand precursors
2a·HBr and 2b·HBr, featuring a pyridine and an imidazolium
moiety, respectively, were synthesized in 1 h and isolated in ex-
cellent yields (> 97%, Table 1, entries 4 and 5). In this case, the
use of NaHCO3 (1.5 equiv) was necessary to deprotonate 2-
(bromomethyl)pyridine hydrobromide and obtain satisfactory
[a] A. Beillard,+ E. Golliard,+ V. Gillet, Dr. X. Bantreil, Dr. T.-X. MØtro,
Prof. Dr. J. Martinez, Dr. F. Lamaty
Institut des BiomolØcules Max Mousseron (IBMM), UMR 5247
CNRS, UniversitØ Montpellier, ENSCM, UniversitØ de Montpellier
Place Eug›ne Bataillon, 34095 Montpellier cedex 5 (France)
E-mail: xavier.bantreil@umontpellier.fr
txmetro@umontpellier.fr
frederic.lamaty@umontpellier.fr
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503472.
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Communication
Table 1. Mechanosynthesis of imidazolium halides.[a]
Entry
Products
R1
Product·HX
Time [h]
Yield [%]
Lit. conditions
1
2
3
4
5
6
7
8
Me
iPr
Me
Me
iPr
Me
Me
iPr
1a·HBr
1b·HBr
1a·HCl
2a·HBr
2b·HBr
2a·HCl
3a·2HBr
3b·2HBr
1
1
2
98 (98)[b]
98
82%, THF, rt, 16 h[10]
unprecedented
78%, solvent-free, 5 h, 70–1108C[11]
87%, CH3CN, 48 h, rt[12]
60%, dioxane, 12 h, 1018C[13]
88%, dioxane, 16 h, 1018C[14]
unprecedented
94
1
1
3.5
1
1
97[b,c]
99[c]
84
88
85
unprecedented
9
10
Me
iPr
4a·2HBr
4b·2HBr
1
2.2
79
93
92%, dioxane, 48 h, 1008C[15]
58%, THF, 7d, 668C[9]
11
12
Y=CH
Y=N
5a·HBr
5b·HBr
1
2.5
94 (99)[b]
79
87%, toluene, 72 h, 708C[16]
72%, EtOH, 48 h, 798C[17]
13
6·HBr
2.5
93
76%, CH3CN, 48 h, 808C[18]
[a] Total mass of reactants = 182 mg; vbm = vibratory ball-mill. [b] Realized in a planetary ball-mill (pbm), with a total mass of reactants = 3977 mg.
[c] Product was recovered by using iPrOH.
results. Less reactive alkyl chlorides were also mechanically
converted into imidazoliums 1a·HCl and 2a·HCl, with slightly
longer reaction times (<3.5 h). When 1,2-bis(bromomethyl)-
benzene and 2,6-bis(bromomethyl)-pyridine were used as the
starting reagents, unreported bis-imidazoliums 3a·2HBr,
3b·2HBr, 4a·2HBr, and 4b·2HBr, were obtained in a very effi-
cient manner. The advantage of mechanochemistry was espe-
cially highlighted with 4b·2HBr, as only 2.2 h of milling was
necessary to isolate this product in 94% yield, while 7 days in
refluxing THF were required for a literature procedure (isolated
in 58% yield).[9] The reaction of imidazole/benzimidazole with
benzyl bromide or 2-(bromomethyl)pyridine hydrobromide in
the presence of NaHCO3 proved that this ball-milling proce-
dure could also be extended to symmetrical imidazolium salts.
Thus, compounds 5a·HBr, 5b·HBr, and 6·HBr were isolated
with improved yields compared with the previous methods
(entries 11–13).
reaction times.[19] The superiority of solvent-free mechanosyn-
thesis over classical solution-based approaches can be ex-
plained by the higher reagent concentrations, which, in combi-
nation with an efficient mixing that avoids mass transfer limita-
tions, results in higher reaction rates. In addition, the majority
of the N,N-dialkylimidazolium mechanosyntheses presented in
this study required at least one liquid reagent. The capacity of
a liquid to present a greater reactive surface area to other re-
agents and to potentially serve as a solvent[20] appears to us as
a good rationale, in this case, to explain the higher efficiency
of this approach. This methodology proved general, as numer-
ous mono-, di-, and tridentate ligand precursors were pro-
duced.
The synthesis of the corresponding silver(I)–carbene com-
plexes was next envisaged by the reaction of imidazoliums 1a–
6·xHX with silver(I) oxide in a vibratory ball-mill under solvent-
free conditions. Thus, metalation of 1a·HBr, which was report-
ed to last 24 h in solution, was completed within only ten mi-
nutes under milling, furnishing [Ag(1a)Br] in 93% yield
(Table 2, entry 1). In this case, the higher efficiency of mecha-
nochemical synthesis over a solution-based approach could be
attributed to the higher rate of creation of new surfaces on
Ag2O particles caused by impacts and shear stress induced by
ball-milling.[20,21] With this efficient method in hand, various
complexes, known or unprecedented, were synthesized. After
milling, the reaction mixtures were recovered with CH2Cl2, fil-
tered over celite and concentrated under vacuum. 1H NMR
Notably, imidazoliums 1a·HBr and 5a·HBr were produced in
similar yields when synthesized in a vbm or in a pbm, showing
that the type of agitation has no influence on the efficiency of
the approach (Table 1, entry 1 and 11). While a solvent-based
synthesis of 1a–6·xHX salts previously described in the litera-
ture required adaptation of the conditions for each compound,
such as long reaction times (up to 7 days), in some cases re-
fluxing solvents, the use of ball-mill and solvent-free conditions
provides a general, user-, and eco-friendly method to obtain
imidazoliums with improved yields and dramatically reduced
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Table 2. Mechanosynthesis of [Ag(NHC)X] complexes.
Entry Product
Ag2O Time
[equiv]
Yield
[%]
Conditions
1
2
3
[Ag(1a)Br] 0.55
[Ag(1b)Br] 0.5
[Ag(1a)Cl] 0.55
10 min 93
10 min 85
10 min 76
91%, CH2Cl2, rt, 24 h[23]
unprecedented
Figure 1. Graphical representation of [{Ag(1b)2}+·AgBr2À]2. H atoms are omit-
ted for clarity.
yield n.a.,[a] CH2Cl2, reflux,
24 h,[24]
4
5
6
7
8
9
10
11
12
13
[Ag(2a)Br] 0.5
[Ag(2b)Br] 0.55
[Ag(2a)Cl] 0.55
[Ag2(3a)Br2] 1.1
[Ag2(3b)Br2] 1.5
[Ag2(4a)Br2] 1.1
[Ag2(4b)Br2] 1.5
[Ag(5a)Br] 0.5
[Ag(5b)Br] 0.6
1 h
1 h
1 h
1 h
1 h
2 h
2.5 h
78
72
71
71%, CH2Cl2, rt, 12 h[12]
unprecedented
We next envisioned to further demonstrate the efficiency of
this novel approach by performing the synthesis of [Ag(NHC)X]
complexes in a one-pot sequential alkylation/metalation proce-
dure starting from imidazoles. This technique was very efficient
for some [Ag(NHC)X] complexes, such as [Ag(1a)Br] and
[Ag(1b)Br] (Table 3, entries 1 and 2). Interestingly, complexes
[25]
82%, CH2Cl2
polymer unprecedented
83 unprecedented
30–75[b] unprecedented
77
88%, CH2Cl2, rt, 24 h[9]
20 min 80
75%, CH2Cl2, rt, 2 h[26]
79%, CH3CN, reflux, 16 h[27]
unprecedented
1 h
1 h
77
71
[Ag(6)Br]
0.5
[a] n.a. = not available. [b] Reaction was not reproducible.
Table 3. A one-pot, two-step generation of silver–NHC complexes.
analysis of the crude mixtures revealed complete conversion of
the substrates and high purity of the products in all cases.
Except for complexes [Ag(2a)Cl] and [Ag2(4b)Br2] (Table 2, en-
tries 6 and 10), all previously described silver(I)–carbene com-
plexes were obtained in similar or improved yields and re-
duced reaction times (entries 3, 4, 11, and 12). This particular
activation method also allowed the synthesis of five unprece-
dented silver(I)–carbene complexes (entries 2, 5, 8, 9, and 13).
Imidazoliums 2a·HX, 2b·HX, and 5b·HBr, which contain at least
one pyridine ring, required longer milling times (up to 2.5 h)
than their phenyl-containing congeners 1a·HX, 1b·HX, and
5a·HBr (<1 h, entries 4–6 and 12 versus 1–3 and 11). The reac-
tion of 3a·2HBr with Ag2O did not yield the expected
[Ag2(3a)Br2] but led to the formation of an insoluble solid,
which was postulated to be an organometallic polymer
(entry 7). Similar issues were also observed for [Ag2(4a)Br2],
which was obtained in irreproducible yields (entry 9). However,
the presence of a more hindered iPr substitution inhibited side
reactions, thus [Ag2(3b)Br2] and [Ag2(4b)Br2] were isolated in 83
and 77% yield, respectively (entries 8 and 10). It is worth
noting that this approach proved to be particularly robust as
high yields were obtained independently from various parame-
ters known to have critical influence in solvent-free or solvent-
less mechanochemical reactions, such as relative humidity,
type and intensity of milling, physical aspects of the reagents,
and reaction mixture.[22]
Entry
Product
Time 1st + 2nd step
Ag2O
[equiv]
Yield
[%]
1
2
3
4
5
6
[Ag(1a)Br]
[Ag(1b)Br]
[Ag(2a)Br]
[Ag2(3b)Br2]
[Ag2(4a)Br2]
[Ag2(4b)Br2]
1 h + 10 min
1.5 h + 50 min
1 h + 2 h
1 h + 2 h
2 h + 2.5 h
2.5 h + 2.5 h
0.55
0.55
0.55
1.5
1.05
1.05
80
99
49[a]
91
<35
86
[a] NMR conversion.
[Ag2(3a)Br2] and [Ag2(4b)Br2] were obtained with improved
yields when synthesized by this one-pot procedure compared
to the conditions reported in Table 2 (entry 4 and 6). However,
the isolation of [Ag2(4a)Br2] was still difficult (vide supra;
entry 5).
The usefulness of mechanochemistry in organometallic syn-
thesis was further demonstrated by reacting [Ag(1a)Br] with
[AuCl(SMe2)] (Scheme 2). The transmetalation occurred within
30 min, yielding complex [Au(1a)Cl] in 85%, thus completing
To confirm the structures of the [Ag(NHC)X] complexes ob-
tained by this original mechanochemical approach, several
compounds were recrystallized in a CH2Cl2/pentane mixture
and single crystals were submitted to X-ray diffraction analysis.
Thus, unprecedented [Ag(1b)Br] was confirmed to be a bis-
NHC silver cationic complex [{Ag(1b)2}+.AgBr2À]2 in which the
À
counter anion AgBr2 forms a dimer (Figure 1).[28]
Scheme 2. Direct and sequential mechanosyntheses of [Au(1a)Cl].
Chem. Eur. J. 2015, 21, 17614 – 17617
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Communication
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a ball-mill with high efficiency. Moreover, [Au(1a)Cl] could also
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each step).
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dialkyl-substituted NHC precursors and their corresponding
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and alkyl halides without the isolation of intermediates, hence,
paving the way for further one-pot multi-step mechanosynthe-
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Acknowledgements
UniversitØ Montpellier and CNRS are acknowledged for fund-
ing. We thank Prof. J.-M. Campagne and the AM2N team for
generously providing [AuCl(SMe2)], and Dr. S. Richeter and the
IMNO team for [AuCl(THT)]. F. MØtro is warmly acknowledged
for the production of the graphical abstract.
Keywords: ball-mill · green chemistry · mechanochemical
reactions
reactions
· N-heterocylic carbenes · silver · solvent-free
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